This invention relates generally to processes for producing electric wires and cables or conductors coated with a crosslinked polyethylene resin. More specifically, the invention relates to a process for producing coated electric wires and cables (hereinafter referred to collectively as coated "wire(s)" or "conductor(s)") which process comprises extrusion-coating an electric conductor with a copolymer of ethylene and an ethylenically unsaturated silane compound having a hydrolyzable silane group as a crosslinkable group, and converting the copolymer into a crosslinked structure.
A generally practiced method of improving the physical properties, particularly heat resistance, of a coated wire having an electric conductor coated with a coating layer comprising a polyolefin such as polyethylene is to cause crosslinking of this polyolefin.
One known measure for carrying out this crosslinking is to render the polyolefin to be used crosslinkable beforehand by introducing thereinto a crosslinkable group. In this case, the utilization of a hydrolyzable silane group as the crosslinkable group is known. More specifically, the process comprises preparing a graft copolymer by reacting the polyolefin and an ethylenically unsaturated silane compound in the presence of a free radical generating agent, extrusion-coating a composition comprising this graft copolymer and a silanol condensation catalyst on an electric conductor thereby to produce a coated wire, and causing this coated wire to contact water thereby to cause the graft copolymer to undergo crosslinking.
While this process is accompanied by a problem in that a special process step for grafting beforehand the unsaturated silane compound is necessary, and a general-purpose polyolefin cannot be used as it is, it is more advantageous than other crosslinking processes such as, for example, that depending on radiation and that comprising compounding an organic peroxide with the polymer beforehand and then decomposing the peroxide. More specifically, the radiation process requires special and expensive apparatus and, in addition, cannot produce coated wires of thick coating in actual practice, while the peroxide process is accompanied by problems such as generation of bubbles in the coating due to decomposition gases, whereby difficulties are encountered in obtaining crosslinked coating layers of uniform quality, particularly in the case of thin coatings.
However, there have been problems in the use of these unsaturated silane compound-grafted polyolefins also, That is, the surface characteristics of products of these graft polyolefins are not necessarily good in all cases. This problem becomes seriously pronounced when the line speed of extrusion is high. However, needless to say, a high extrusion line speed is desirable from the industrial viewpoint. Consequently, this problem of poor surface characteristics including rough surface texture is not negligible.
This problem of poor surface characteristics is serious, not only on the point merely of commodity value of the product, but also on the point, for example, of the insulation characteristic of an insulated wire which is the product in the case where, over the crosslinked coating layer, a further coating layer is applied to form an insulating layer, the surface of which must be smooth for good insulation characteristic. These surface characteristics depend on the degree of crosslinking and can be improved by a lowering of the degree of crosslinking, but this gives rise to a simultaneous deterioration of the heat resistance of the coating. Accordingly, this improvement measure cannot be resorted to. Furthermore, the use of these graft polyolefins has given rise to a problem in that, in continuous extrusion forming of the coated wire over a long period, the quantity of the extruded resin decreases with the passage of time, which in turn may lead to trouble such as impossibility of extrusion.